It is becoming increasingly common practice to connect the leads extending from an electrical device to conductors on a circuit board by mounting the electrical device on the same surface of the circuit board as the surface upon which the circuit board conductors are provided. This surface mounting technique is carried out by providing terminal pads or areas on the circuit board conductors and contact portions on the leads extending from the electrical device which are positioned against the terminal pads. The pads are coated with a viscous solder composition and after placement of the electrical device on the surface of the circuit board, the solder composition is reflowed thereby to obtain the bond between each lead and its associated terminal pad.
One prior art connector is shown in U.S. Pat. No. 4,210,376 as having a plurality of electrical terminal portions for interconnection to printed circuit board through holes. The terminal portions of the connector are arranged in two spaced apart rows along the back side of the housing to position the lead ends into two staggered rows of terminals. This connector is also available in a surface mount version where the lead ends are bent upwardly for contact on the surface of the printed circuit board rather than through a hole in the printed circuit board.
There are several problems which must be addressed when the more conventional terminal post and circuit board hole mounting technique is replaced by a surface mounting technique. For example, the contact portions of the leads extending from the electrical device should be precisely located in coplanar relationship so that when the electrical device is positioned on the circuit board in preparation for the soldering operation, the contact portion of every one of the leads will be against the corresponding terminal pad on the circuit board. If one or more of the leads is not coplanar, the connector will have to be forced towards the board, to a position where all contact portions are contacting their respective surface mount pads. Successful execution of the reflow soldering operation requires that a certain amount of force be placed on the terminal pads through the contact portions of the terminals, such as through resilient deflection of the terminals when the connector is placed on the board. If the contact portions are not coplanar, the contacts which are in a plane closer to the board, will require a greater deflection than those contact portions which are spaced from the board, such that all contact portions mate with their respective pads on the surface of the board. This increased deflection on some of the terminals causes a proportionate increased amount of biasing force against the lead contact portions.
This increased force can cause the connector itself to unseat itself from the printed circuit board. The connector typically includes a retaining feature on the mounting face thereof, such as that shown in U.S. Pat. No. 4,195,900, incorporated herein by reference, which is profiled for receipt through apertures in the printed circuit board for retaining the connector to the housing. An increased biasing force in opposition to the retention member can cause the retention features to give way to the biasing force, unseating the connector housing from the surface of the printed circuit board.
One other difficulty which is encountered with a surface mount connector similar to the above described, relates to the side-to-side alignment of the lead ends, to ensure precise alignment of the lead ends on the printed circuit board. The lead ends are spaced at 0.050 inches between centers, thus precise alignment of the lead ends is required to ensure that the leads ends contact the printed circuit board pads, and that the lead ends do not contact each other.
It must also be recognized that surface mounted electrical devices inherently require more space on the circuit board than does a device mounted by means of terminal posts and circuit board holes for the reason that the leads must extend laterally a short distance away from the insulating body, such as a connector housing, of the electrical device. This requirement of additional circuit board space for surface mounted devices can be minimized but it cannot be eliminated. It is also desirable to mechanically isolate the soldered connections of the contact portions of the leads to the terminal pads of the circuit board from the insulating body of the electrical device in order to avoid or minimize the transmission of stresses from the device to the soldered joints.
The present invention is directed to the achievement of an improved surface mountable electrical device having leads extending therefrom which satisfy the requirements discussed above.